The present invention relates generally to diagnostic imaging and, more particularly, to a method and system of thermoacoustic computed tomography (TCT) with exact inversion of TCT data for image reconstruction.
It is generally well known that wave propagation and integral geometry are the physical and mathematical underpinnings of most diagnostic imaging modalities. To date, most of these standard modalities have been predicated upon the measurement of the same type of output energy as was input to the system. For example, ultrasound diagnostic systems transmit and receive ultrasonic waves and, from those ultrasonic waves, are capable of generating a diagnostic image. CT systems are predicated upon the transmission and reception of x-ray or gamma ray radiation. In conventional CT systems, x-rays are projected toward an imaging subject and the attenuation of those x-rays caused by the subject is measured and processed to reconstruct a diagnostically valuable and probative image of the subject. Recently, however, hybrid imaging or diagnostic systems have been developed along with associated imaging techniques whereupon the measured output energy is different in form and type from the energy input to the system.
For instance, thermoacoustic tomography (TCT) is predicated upon and uses radio frequency (RF) energy projected at an imaging subject and measures emitted ultrasonic waves resulting from the application of the RF energy. Near infrared radiation is also non-ionizing and may also be used to heat tissue. TCT imaging involves the measurement of ultrasonic signals that are induced in the tissue of a subject whenever pulsed or continuous application of radiation is absorbed within the tissue, and the detection of resulting ultrasonic signals with transducers placed on or outside the imaging subject. More particularly, the ultrasonic transducers placed about the subject detect shock waves that are created in tissue when RF energy is absorbed and cause a heating and expansion of tissue. For example, it is known that cancerous masses absorb more RF energy than healthy tissue. As such, cancerous masses preferentially absorb RF energy, heat, and expand more quickly than neighboring healthy tissue thereby creating a shock wave which, when detected by an ultrasonic transducer, allows for detection of, or contrast between, cancerous or abnormal tissues and healthy tissues. Therefore, assuming a constant sound speed, the sound or ultrasonic waves, detected at any point in time after application of the RF energy, are generated by inclusions or abnormal masses lying on a sphere of radius cΔt where Δt=time delta between RF input and received signal centered at a particular transducer. Standard reflection ultrasound assumes constant soundspeed. Because this assumption is correct to within a few percent, the surfaces of integration are nearly spherical.
Known TCT relies upon an inexact inversion solution to generate a reconstruction data set. This inexactitude of the reconstruction data can therefore affect image quality and, ultimately, the diagnostic value of the reconstructed image. More precisely, standard reconstruction method used in TCT is filtered backprojection with xray CT weights and kernel, even though such a method is not exact for TCT.
It would therefore be desirable to design a method and system of TCT imaging whereupon the acquired TCT data is reconstructed via a mathematically exact method.